Magnetic field sensors (e.g., rotation detectors) for detecting ferromagnetic articles and/or magnetic articles are known. The magnetic field associated with the ferromagnetic article or magnetic article is detected by a magnetic field sensing element, such as a Hall element or a magnetoresistance element, which provides a signal (i.e., a magnetic field signal) proportional to a detected magnetic field. In some arrangements, the magnetic field signal is an electrical signal.
The magnetic field sensor processes the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses thresholds, either near to peaks (positive and/or negative peaks) or near to some other level, for example, zero crossings of the magnetic field signal. Therefore, the output signal has an edge rate or period indicative of a speed of rotation of the ferromagnetic or magnetic object, for example, a gear or a ring magnet.
One application for a magnetic field sensor is to detect the approach and retreat of each tooth of a rotating ferromagnetic gear, either a hard magnetic gear or a soft ferromagnetic gear. In some particular arrangements, a ring magnet having magnetic regions (permanent or hard magnetic material) with alternating polarity is coupled to the ferromagnetic gear or is used by itself and the magnetic field sensor is responsive to approach and retreat of the magnetic regions of the ring magnet. In other arrangements, a gear is disposed proximate to a stationary magnet and the magnetic field sensor is responsive to perturbations of a magnetic field as the gear rotates.
In one type of magnetic field sensor, sometimes referred to as a peak-to-peak percentage detector (or threshold detector), one or more threshold levels are equal to respective percentages of the peak-to-peak magnetic field signal. One such peak-to-peak percentage detector is described in U.S. Pat. No. 5,917,320 entitled “Detection of Passing Magnetic Articles While Periodically Adapting Detection Threshold” and assigned to the assignee of the present invention.
Another type of magnetic field sensor, sometimes referred to as a slope-activated detector (or peak-referenced detector, or peak detector for short), is described in U.S. Pat. No. 6,091,239 entitled “Detection Of Passing Magnetic Articles With a Peak Referenced Threshold Detector,” also assigned to the assignee of the present invention. In the peak-referenced magnetic field sensor, the threshold signal differs from the positive and negative peaks (i.e., the peaks and valleys) of the magnetic field signal by a predetermined amount. Thus, in this type of magnetic field sensor, the output signal changes state when the magnetic field signal comes away from a peak or valley of the magnetic field signal by the predetermined amount.
It should be understood that, because the above-described threshold detector and the above-described peak detector both have circuitry that can identify the positive and negative peaks of a magnetic field signal, the threshold detector and the peak detector both include a circuit portion referred to as a “peak identifier” herein, which is configured to detect positive peaks and/or negative peaks of the magnetic field signal. The threshold detector and the peak detector, however, each use the detected peaks in different ways to provide a so-called “threshold generator,” which is configured to use the identified peaks to generate one or more thresholds against which the magnetic field signal can be compared. This comparison can result in a so-called “PosComp” signal that has an edge rate representative of a speed of movement, e.g., rotation, of the moving object.
In order to accurately detect the positive and negative peaks of a magnetic field signal, in some embodiments, the rotation detector can be capable of tracking at least part of the magnetic field signal. To this end, typically, one or more digital-to-analog converters (DACs) can be used to generate a tracking signal, which tracks the magnetic field signal. For example, in the above-referenced U.S. Pat. Nos. 5,917,320 and 6,091,239, two DACs are used, one (PDAC) to detect the positive peaks of the magnetic field signal and the other (NDAC) to detect the negative peaks of the magnetic field signal.
Some types of rotation detectors perform one or more types of initialization or calibration, for example, at a time near to start up or power up of the rotation detector, or otherwise, from time to time as desired. During one type of calibration, the above-described threshold level is determined.
Some moving objects, for example, rotating moving objects, which are sensed by the above-described magnetic field sensors, exhibit irregular motions or have irregular features. For example, a gear may have wobble as it rotates, it may have run out (asymmetry about its axis of rotation), or it may have irregularities in its mechanical dimensions, for example, some gear teeth may be wider than others. These irregularities tend to result in generation of thresholds that are not ideal. The non-ideal threshold tends to result in a PosComp signal that has edges that are not accurately placed relative to cycles of the magnetic field signal associated with the moving object.
It would, therefore, be desirable to provide a magnetic field sensor that can accurately identify a threshold level associated with a magnetic field signal, accurate even in the presence of irregularities in the motion of, or in the mechanical characteristics of the moving object being sensed.